1. Field of the Invention
The present invention relates generally to a multihop relay broadcast wireless access (BWA) communication system, and in particular, to an apparatus and method of providing transparent relay service to a mobile station in a relay supporting system for cell capacity expansion of a serving cell.
2. Description of the Related Art
In fourth generation (4G) communication systems, research has been conducted to provide users with various Quality of Service (QoS) at a data rate of over 100 Mbps. Specifically, research into the high rate service support to guarantee mobility and QoS in broadband wireless access (BWA) communication systems, such as Local Area Networks (LAN) and Metropolitan Area Networks (MAN), has been under way. Representative systems of the BWA communication system include Institute of Electrical and Electronics Engineers (IEEE) 802.16d and 802.16e communication systems.
IEEE 802.16d and 802.16e communication systems adapt an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme for physical channels. An IEEE 802.16d communication system addresses the stationary or fixed Subscriber Station (SS); that is, an IEEE 802.16d communication system does not take into account the mobility of the SS but the single cell structure. By contrast, an IEEE 802.16e communication system addresses the mobility of the SS, which is updated from the IEEE 802.16d communication system. The mobile SS is referred to as a Mobile Station (MS).
FIG. 1 shows a general IEEE 802.16e communication system which has a multi cell architecture, that is, a cell 100 and a cell 150. The IEEE 802.16e communication system includes a Base Station (BS) 110 managing the cell 100, a BS 140 managing the cell 150, and a plurality of MSs 111, 113, 130, 151, and 153. Signals are transmitted and received between the BSs 110 and 140 and the MSs 111, 113, 130, 151, and 153 using an OFDM/OFDMA scheme. Of the MSs 111, 113, 130, 151, and 153, the MS 130 resides in the overlapping area of the cell 100 and the cell 150; that is, in a handover region. When the MS 130 migrates to the cell 150 managed by the BS 140 while transmitting and receiving signals to and from the BS 110, the serving BS of the MS 130 is changed from the BS 110 to the BS 140.
By signaling through the direct links between the fixed BS and the MSs as shown in FIG. 1, the general IEEE 802.16e communication system can easily configure highly reliable wireless communication links between the BS and the MSs. However, since the position of the BS is fixed, the IEEE 802.16e communication system has low flexibility in the radio network configuration. Thus, it is hard to provide efficient communication services in a radio communication environment suffering severe changes of traffic distribution or traffic demand.
To overcome these shortcomings, a data delivery scheme using a multihop relay having a fixed relay station, a mobile relay station, or general MSs is applicable to a general cellular wireless communication system, such as an IEEE 802.16e communication system. A wireless communication system using a multihop relay scheme can reconfigure the network by promptly coping with the changes of the communication environment and utilize the entire radio network more efficiently. For instance, a multihop relay wireless communication system is able to expand the cell service area and increase system capacity. In detail, under poor channel conditions between a BS and an MS, better radio channel status can be provided to the MS by installing a relay station between the BS and the MS and establishing a multihop relay path via the relay station. Also, by adopting a multihop relay scheme in a cell boundary of a poor channel status from the BS, a high speed data channel can be provided and the cell service area can be expanded.
Hereinafter, descriptions are provided on a structure of a multihop relay wireless communication system for expanding the service area of a BS.
FIG. 2 shows a broadband wireless communication system using a multihop relay scheme for extending the service area of the BS. The multihop relay wireless communication system in FIG. 2 has a multicell architecture, that is, a cell 200 and a cell 240. The multihop relay wireless communication system includes a BS 210 managing the cell 200, a BS 250 managing the cell 240, MSs 211 and 213 located in the cell 200, MSs 221 and 223 managed by the BS 210 but located in an area 230 out of the cell 200, a relay station 220 providing multihop relay paths between the BS 210 and the MSs 221 and 223 in the area 230, MSs 251, 253 and 255 located in the cell 240, MSs 261 and 263 managed by the BS 250 but located in an area 270 out of the cell 240, and a relay station 260 providing multihop relay paths between the BS 250 and the MSs 261 and 263 in the area 270. Signals are transmitted and received among the BSs 210 and 250, the relay stations 220 and 260, and the MSs 211, 213, 221, 223, 251, 253, 261, and 263 using an OFDM/OFDMA scheme.
The MSs 211 and 213 and the relay station 220, which belong to the cell 200, can transmit and receive signals directly to and from the BS 210, whereas the MSs 221 and 223 in the area 230 can not transmit and receive signals directly to and from the BS 210. Hence, the relay station 220 manages the area 230 and relays signals between the BS 210 and the MSs 221 and 223 which are incapable of transceiving signals directly. The MSs 221 and 223 can transceive signals with the BS 210 via the relay station 220. Likewise, the MSs 251, 253 and 255 and the relay station 260, which belong to the cell 240, can transmit and receive signals directly to and from the BS 250, whereas the MSs 261 and 263 in the area 270 can not transmit and receive signals directly to and from the BS 250. Hence, the relay station 260 manages the area 270 and relays signals between the BS 250 and the MSs 261 and 263 which are incapable of transceiving signals directly. The MSs 261 and 263 can transmit and receive signals to and from the BS 250 via the relay station 260.
FIG. 3 shows a broadband wireless communication system using a multihop relay scheme for expanding the system capacity. The multihop relay wireless communication system in FIG. 1 includes a BS 310, MSs 311, 313, 321, 323, 331 and 333, and relay stations 320 and 330 which provide multihop relay paths between the BS 310 and the MSs 311, 313, 321, 323, 331 and 333. Signals are transmitted and received among the BS 310, the relay stations 320 and 330, and the MSs 311, 313, 321, 323, 331 and 333 according to the OFDM/OFDMA scheme. The BS 310 manages a cell 300. The MSs 311, 313, 321, 323, 331 and 333 and the relay stations 320 and 330, belonging to the cell 300, are capable of transmitting and receiving signals directly to and from the BS 310.
However, when some MSs 321, 323, 331 and 333 reside close to the boundary of the cell 300, a Signal-to-Noise Ratio (SNR) of the direct links between the BS 310 and the some MSs 321, 323, 331 and 333 may lower. Thus, the relay station 320 relays the unicast traffics of the BS 310 and the MSs 321 and 323, and the MSs 321 and 323 transmit and receive the unicast traffics to and from the BS 310 via the relay station 320. Likewise, the relay station 330 relays the unicast traffics of the BS 310 and the MSs 331 and 333, and the MSs 331 and 333 transmit and receive the unicast traffics to and from the BS 310 via the relay station 330. That is, the relay stations 320 and 330 raise the effective data rate of the MSs and increase the system capacity by providing high-speed data delivery paths to the MSs 321, 323, 331 and 333.
In a broadband wireless communication system using a multihop relay scheme of FIG. 2 or 3, the relay stations 220, 260, 320 and 330 may be infrastructure relay stations which are installed by a service provider and already known to the BSs 210, 250 and 310 for management, or client relay stations which serve as subscriber stations (e.g., SSs or MSs) in some cases and relay stations in other cases. The relay stations 220, 260, 320 and 330 may be fixed relay stations, nomadic relay stations (e.g., notebook computers), or mobile relay stations such as MSs.
As discussed above, in a multihop relay wireless communication system, an RS is a relay station for cell expansion, which supports the relay between an MS outside a BS region and a BS, or a relay station for cell capacity increase, which supports the relay between an MS within the BS region and the BS.
In the mean time, when utilizing an RS for cell area expansion or a cell capacity increase, a relay service of the RS can be provided while not informing the MS of the presence of the RS; that is, by making the MS recognize the data transmission and reception with the BS. This is referred to as a transparent relay. To provide a transparent relay to the MS, a new signaling process should be defined between the BS and the RS. Primarily, it is desired to define a signaling process of selecting an RS.